Reactor for bulk production of photosynthetic microorganisms

ABSTRACT

An algae production reactor system according to the invention comprises a reactor vessel which is provided with:—one or more liquid inlets and one or more liquid outlets;—one or more gas inlets at the bottom, said gas inlets being connected with a source of carbon dioxide, and one or more gas outlets at the top of the vessel;—vertically interspaced and joined pairs of double glass plates which are at least partially submerged in the reactor liquid, said double glass plates having a layer of light-scattering particles in between and having a flat side being exposed to a light source; and—means for vertically circulating reactor liquid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is Continuation Application of U.S. patent applicationSer. No. 14/778,523, filed Sep. 18, 2015, which is the National Phase ofInternational Patent Application No. PCT/NL2014/050172, filed Mar. 21,2014, published on Sep. 25, 2014 as WO 2014/148903 Al1, which claimspriority to European Application No. 13160450.6, filed Mar. 21, 2013.The contents of which are herein incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates to the production of algae and isparticularly concerned with the growth of algae in an enclosed reactorsystem where the sunlight can be fed into the reactor by a mirrorsystem. The invention provides a reactor vessel and light-introducingmeans for such an algal production reactor system.

BACKGROUND

Algae use light and carbon dioxide for growing and this processgenerates oxygen. Normally algae are produced in open ponds, transparentpiping systems, submerged plastic bags, etc.

The disadvantages of these systems are the amount of energy necessaryfor mixing, feeding the carbon source and removal of the generatedoxygen. Also the amount of water evaporation, heating of the water indaytime and the cooling in the night is a problem. This is because theonly sensible source of light for growing algae is sunlight, which is inprinciple available for free, but for optimum growth, the algae requirea high influx of light. However, sites where high sunlight input isavailable are almost inherently hampered by being located in arid zonesof the earth, where water, also necessary for algal production, is veryscarce.

These disadvantages limit the algae concentration in the reactor systemsand increase thus the amount of reactor space necessary for a specificamount of algae production. Thus there is need for more efficacious andup-scalable reactor vessels for growing photosynthetic microorganismswith improved light supply.

JP-A 2000-300244 discloses a photosynthetic culturing device havinglight-transmitting plates made of acrylic, which are arranged verticallyin a reactor, the (sun)light entering on the top of the reactor. Thedistance of 10-70 mm between the plates provides the reactor spacecontaining the culture medium in an up-flow arrangement. The top ends ofthe spaces between the plates are closed with covers (“first invention”)or with triangular extensions of the acrylic plates for increasedirradiation surface (“second invention”). The light-transmitting platesmay have light-scattering surfaces provided by unidirectional striping,and pairs of plates may be formed in such a way that thelight-scattering surfaces are at the inner side of the pairs, asdescribed in JP-A H08-262232.

The reactors of the types descried in JP-A 2000-300244 and JP-AHOS-262232 do not provide for optimum irradiation efficiency and algalproduction rates. Moreover, the arrangement nature of the plates usedaccording to this prior art do not allow the provision of robust, largescale photosynthesis reactors for use at remote sites and requiringminimum maintenance operations.

SUMMARY OF THE INVENTION

It was found according to the invention, that improved photosynthesiscan be provided by means of a reactor of an airlift-loop type, usinglateral irradiation, and by providing unit pairs of double rectangularglass plates which are mounted in parallel at a distance provided byglass strips arranged between the rectangular double glass plates andalong the long (vertical) sides of the double glass plates. The use ofglass plates rather than acrylic-type plates reduces fouling bymicroorganisms. Further improvement is achieved by providinglight-scattering layers at the inside of the double glass plates, thelight scattering layers comprising non-unidirectional layers ofnon-uniformities or particles.

The invention thus provides a large-scale reactor for growing algaewhich has an upward flow and a downward flow channel, which canalternate e.g. by switching the feeds of the gas mixing flow. Gas caninduce the upward and downward flow circulation. These types of airliftloop reactors are known as such in waste water treatment. Vertical glassplates are arranged in the upward and in the downward flow. The frontparts of these glass plates are sticking out of the reactor wall into a(sun)light source, together with glass strips joining pairs of doubleglass plates along their long (vertical) sides. These glass plates areused to transfer light into the reactor which is necessary for algalgrowth inside the reactor.

DESCRIPTION OF THE INVENTION

The invention therefore pertains to a reactor vessel for the productionof photosynthetic microorganisms. Such a reactor is also referred to byits synonyms “photosynthesis reactor” or “photobioreactor”. The reactorvessel is provided with one or more liquid inlets and one or more liquidoutlets, one or more gas inlets at the bottom and one or more gasoutlets at the top of the vessel, and vertically interspaced sets ofdouble glass plates which are at least partially submerged in thereactor liquid when the reactor vessel is in operation. The double glassplates have a layer of light-scattering particles in between and have aflat side corresponding to the thickness of the double glass plates,which flat side is being exposed to an external light source. Inoperation, the flat sides are suitably vertically. The reactor vessel isfurther provided with means for vertically circulating reactor liquid.The reactor as ready for operation also contains the further means andmaterials for producing algae by photosynthesis, including a suitableinoculate of the algae to be produced, a source of carbon dioxide, etc.

The double glass plates, which are arranged pair-wise, constitute animportant element of the reactor vessel of the invention and of theinvention itself The refractive index of the glass is higher than therefractive index of water, which makes the glass plates function like aglass fibre used for data transport. The layer of light-scatteringparticles ensures that the light leaks out evenly over the submergedarea of the glass plate into the reactor liquid. For this purpose theglass plates consist of two layers (referred to as “double glassplates”) with a coating in between which constitutes a matrix for small(i) inorganic (e.g. metal oxide) or (ii) organic particles or (iii)non-uniformities to scatter the light, or for (iv) larger mirror facetsto reflect the light in such a way that the light can enter the waterface.

The (single) glass plates can have a thickness varying from a few mm,e.g. 5 mm, up to about 50 mm or even more. Advantageously, the thicknessof the glass plates is between 10 and 30 mm, most preferably between 12and 20 mm. Thus, the double glass plates preferably have a thicknessbetween 20 and 60 mm, most preferably between 24 and 40 mm. Similarly,the flat sides of the double glass plates also preferably have a breadthof 20-60, or 24-40 mm. The glass is preferably of the so-called“ultra-clear” type, i.e. it has a high clarity and is low in iron, inparticular below 0.04 wi:.% (as Fe₂O₃). Such glasses are also calledlow-iron glass, or high transmittance glass.

The layer of light-scattering particles can be a coating layer in whichthe scattering particles are mixed in a material having the same ornearly the same refractive index as glass, so that light can pass fromglass to coating without a mirror effect caused by a difference inrefractive index. For instance it can be a ceramic coating such assilica. In the coating layer non-uniformities can be mixed which causethe scattering (for instance crystallites, bubbles etc.).Non-uniformities are understood to be bodies or voids which constitute adiscontinuity in the matrix, i.e. which can be, at leastelectromagnetically, distinguished from the surrounding coating, forexample by their refractive index. The non-uniformities are preferablysubstantially spherical.

Two types of scattering can be used: (1) Mie-scattering, withnon-uniformities or particles with about the same or slightly larger(average) size than the wavelength of visible light, preferably in therange of 200-1200 nm, more preferably in the range from 300 up to 1000nm, most preferably at least 400 nm, up to e.g. 800 nm; (2) Geometricscattering, with particles much larger than the wavelength of light,preferably in the range from 5 micrometer up to 500 micrometer, forinstance chromium crystals.

The Mie scatterers (non-uniformities) may be metal oxides e.g. oftitanium, zinc, silicon, or aluminium, or silicates, e.g. of magnesiumor aluminium. Alternatively, the Mie scatterers may be organicparticles, e.g. polymer (latex) particles, or they may be (pores,bubbles) of the same size. Particles of metal oxides, in particulartitanium dioxide, are particularly preferred, e.g. of 0.3 to 1 μm.

The geometric scatterers are facetted (mirroring) particles, i.e.particles having light-reflecting facets, such as in chromium crystalsor mono crystalline micro diamonds (6-20 μm). They preferably have sizesin the range of 5-500 μm, more preferably 10-200 μm, in particular20-200 μm.

The thickness of the internal coating containing the light-scatteringnon-uniformities or particles depends on the type of scattering. ForMie-scattering it is preferably in the range of 5 to 500 μm, especiallybetween 10 and 200 μm, and for the geometric scattering it is preferablybetween 100 and 1000 nm. The particles are advantageously homogeneouslydistributed in the internal coating. Their density may be e.g. between 1and 500 mg per dm² of the coating (matrix).

The distance between pairs of double glass plates, i.e. the effectivereactor space, may be as close as e.g. 10 mm, up to, say 20 cm or more.For an optimum light irradiation, it is preferred that the distance isbetween about the thickness of the double plates and about twice saidthickness, i.e. between 20 and 120 mm, most preferably between 24 and 80mm. The number of double glass plates over the width of the reactor mayvary. Preferably the reactor vessel, or reactor unit, contains between 4and 25, more preferably between 6 and 20 double glass plates per m ofreactor width, the glass plates being essentially mounted in parallel.

According to the invention, the double glass plates are advantageouslyarranged in pairs, with strips of glass joining the pairs along thelong, vertical side of the plates, as further described below, and asdepicted in FIG. 3.

The invention also pertains to double glass plates (pairs of glassplates with internal coating) as described above as well as to sets andarrays of double glass plates which can be used in photosyntheticreactors. The double glass plates preferentially have an essentiallyrectangular surface. The length (or height if positioned vertically) ofthe glass plates can be e.g. between 1 and 4 m, and preferably between1.2 m and 2.4 m. The width of the glass plates can be e.g. between 0.5and 2.5 m, preferably between 0.8 m and 2 m. Smaller surfaces may besuitable for pilot-type reactors. Larger surfaces may be feasible aswell, although the weight and the handling of the glass plates may thenrequire special measures. The thickness of the double glass plates canbe as described above, i.e. between 20 and 60 mm, more preferablybetween 24 and 40 mm. The double glass plates have a layer oflight-scattering particles in between, as described above.

Advantageously, two or more, in particular two, double glass plates aremounted together in parallel at a distance allowing an optimum reactorspace between the two plates of from 10 to 200 mm, preferably from 20 to150 mm or more preferably from 24 to 80 mm or even from 30 to 60 mm. Thedistance can be fixed by glass strips being positioned in between andalong the length of the rectangular double glass plates. The strips thusessentially have the same length as the double glass plates, for examplebetween 1 and 4 m. They may also have the same width, for examplebetween 20 and 60 mm. Alternatively, they may have a width of e.g. 1-10times, especially 2-4 times the thickness of the double glass plates, oralternatively 0.01 and 0.2 times, preferably 0.05-0.12 times the breadth(width) of the glass plates (for each strip). Thus a preferred stripwidth is e.g. between 2 and 20 cm, preferably between 2 and 15 cm orbetween 2 and 12 cm or in particular between 2.4 and 8 cm.Preferentially, the strips create a channel for algal reactor medium ofthe same size as the glass plate assembly, so the light exposure of thealgal medium is the same in-between the glass assemblies as in the glassassembly.

The glass strips are preferably of the same glass type as the glassplates and are fixed to the glass plates using an appropriate sealant,such as a polymethacrylate sealant. The sets of two (or more) mounteddouble glass plates provide higher strength of the glass plates, andthus more convenient handling. The reactor vessel, or reactor unit, maythus advantageously contain between 2 and 12, more preferably between 4and 10 of such fixed sets of double glass plates per m of reactor width,the sets being essentially mounted in parallel.

For facilitating the positioning and replacement of glass plates in thereactor vessel, sets of glass plates can advantageously be combined toan array of double glass plates or of sets of double glass platesdescribed above. Thus, e.g. from 4 to 40, especially from 10 to 24equidistant double glass plates, or from 2 to 20, especially from 5 to12 equidistant sets of double glass plates, can be mounted together in asteel rack covered with a suitable, preferably flexible and/orcompressible material, e.g. a thermoplastic polymer such aspolypropylene or polyethylene, in such a way that the glass does notcome into contact with the steel construction. The glass plateassemblies may be provided with suitable supports and guiding stripsallowing the arrays to be mounted in the reacor vessel and combined ashorizontal and/or vertical stacks. A useful stack of arrays of glassplates can e.g. be 3-6 racks stacked vertically and 2-5 rackshorizontally.

The reactor dimensions are preferably such that a sufficient growthefficacy can be achieved through vertical circulation, while avoidingexcessive internal pressures. It was found that reactor liquid heightsof between 3 and 12 m, preferably between 5 and 10 m, most preferablybetween 7 and 9 m, provide an optimum result. Reactor widths(diameters), i.e. in the direction of the incoming light, are limited bythe transmittance of light through normal ultra-clear float glass andtypically not more than 2.5 m, preferably not more than 2.0 meter;suitable reactor widths are e.g.

between 1 and 1.5 m. If desired, multiple reactor vessels can bestacked, so as to further upscale and economise the productionperformance. It is preferred that the reactor is made of steel, inparticular coated steel or stainless steel.

In an advantageous embodiment, the reactor liquid can be provided withfoam objects, which are moving with the circulating reactor liquid inorder to clean the light emitting surfaces of the glass plates. Examplesare sponge-type materials, for instance sponges made of melamine resin,in the size from e.g. 10 mm up to 30 mm, introduced at the bottom of thereactor in the up-flow part and collected at the top of the reactor. Inoperation of the reactor, the up-flow part and the down-flow part areregularly switched thus allowing both parts to be cleaned.

The means for vertically circulating reactor liquid are convenientlyprovided by the gas inlet means being arranged vertically below part ofthe spaces between the sets of glass plates. In particular, about halfof the spaces between the glass plates is positioned above a part of thebottom of the reactor vessel which is provided with gas inlets, thusproviding the upward flowing part of the reactor liquid, and the otherhalf is positioned above a part of the bottom of the reactor vesselwhich is not provided with gas inlets, thus providing the downwardflowing part of the reactor liquid. Advantageously, the gas inlets areprovided in lines below half of the spaces between the double glassplates. The up-flow and down-flow compartments may be arrangedalternatingly, or there may be a number of up-flow spaces followed by anumber of down-flow spaces.

The light source comprises mirrors mounted on a sun tracking systemreflecting sunlight to the flat sides of the glass plates. In this way,sunlight is reflected under a constant angle on the exposed part of theglass plates sticking in the gas lift loop channels of the reactorduring the day.

In order to reduce the heating effect of the sunlight, the light sourcesubstantially only transmits light wavelengths between 400 and 700 nm.This means that the light intensity (in candela) of light havingwavelengths below 400 nm and above 700 nm arriving at the glass platesis less than 50%, preferably less than 20% of the light intensity oflight having a wavelength between 400 and 700 nm. For that purpose, themirrors can advantageously be provided with a coating in such a way thatthe light wavelengths outside the 400 and 700 nm range are absorbed bythe coating and thus not transmitted to the reactor. The skilled personwill be able to select the appropriate UV absorbers and IR absorbersfrom commercially available alternatives.

The gas which is fed into the reactor vessel should comprise carbondioxide needed for growth and photosynthesis of the algae. The gassource contains at least 0.5,-'Q (by vol.) of CO₂, preferably at least5%, more preferably at least 10%, most preferably at least 30% (by vol.)of CO₂, the remainder being any gas, in particular nitrogen. The sourceof carbon dioxide can be a combustion gas originating from afuel-combusting furnace, or a liquid or solid CO₂ source, or the like.Nutrients, such phosphorus, nitrogen, potassium and micronutrients, maybe added to the reactor as known in the art.

The invention also pertains to a process of producing algal products,such as algal oils, comprising culturing algae in a reactor vessel asdescribed above and operated as an airlift loop reactor as known in theart per se. All photosynthetic microorganism, including green algae, redalgae, cyanobacteria, etc., can be grown in the reactor vessel of theinvention, using process parameters as known in the art. The algae canbe harvested and isolated, or the algal products, especially fats foruse as energy source “biodiesel” or as food source (long-chainpolyunsaturated fatty acids), can be collected without the algae beingisolated. Other commonly known algal product that can be produced andisolated by the process of the invention include algal proteins andcarbohydrates, carotenoids and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a section in the vertical plane of thereactor showing the principal parts thereof. A reactor vessel (1) isused for growing the algae. It has an up-flow channel (2) and adown-flow channel (3), which can alternate by switching the feeds (4)and (5) of the gas mixing flow (6). The up-flow and down-flow areseparated by a vertical plate (7), the water level in the vessel isindicated with (8). To make this reactor suitable for algae growth,vertical glass plates (9) are installed in the up-flow and in thedown-flow channels. Foam-like objects (10) move upward in the up-flowzone for cleaning the glass plates.

FIG. 2 shows a section in the horizontal plane of the reactor with thelight mirror system. The vertical pairs of glass plates (9) in theup-flow zone (2) and down-flow zone (3) are shown. The front parts (11)of these plates are protruding from the reactor wall (12) into the lightsource (13). These glass plates (9) are used to transfer light into thereactor which is necessary for growth of the algae (14) inside thereactor (1). The glass plate consists of 2 layers (15) and (16) with acoating (17) in between, in which small metal parts are mixed thatfunction as mirror facets to scatter the light in such a way that thelight can enter the water.

A set of mirrors (18) is mounted on a sun-tracking system (19) toreflect the sunlight during the day under a constant angle on theexposed part of the glass plates sticking in the gas-lift-loop channelsof the reactor. To reduce the heating effect of the sunlight, themirrors (18) are provided with a coating (20) in such a way that onlythe wavelengths between 400 and 700 nm, necessary for algal growth, arereflected to the reactor.

FIG. 3 shows a set of double glass plates in more detail. The set ofglass plates (21) is composed of two double glass plates (9). The set ofglass plates contains 4 glass plates (15,16 twice) with layers (17) ofscattering particles in between. Two glass strips (22,23) are positionedand sealed between the inner glass plates.

FIG. 4 shows a rack containing an array of double glass plates as ahorizontal section (or top view). Sets of double glass plates (21) aremounted in a rack having side walls (33) and front and back wallscomprising spacing strips (34) between the sets of glass plates.

FIG. 5 shows the rack in perspective. The rack (30) comprises side walls(33) and strips (34) for spacing the glass plates as front and backwalls. The rack has an upper part (31) and a lower part (32), forsupporting the glass plates, and for allowing the racks to be stacked. Adetachable device (35) for hoisting the rack into and out of the reactorvessel is mounted at the top of the rack.

FIG. 6 shows a stacked reactor (40) containing eight racks (30) asdepicted in FIG. 5. The stacked reactor has a bottom part (41)containing gas distributors driving the airlift loop (not shown) fed bygas pipes (42) connected to a gas supply (43), liquid/product lines anda liquid/product exit (not shown), and a top part (46) containing liquidinlets (not shown) and a gas exit (47). A three-way valve (44) betweengas supply (43) and gas lines (42) allows to change the gas flow fromone compartment to the other thus changing the riser part to downer partand vice versa. Guiding strips (45) supported by carrier strips (48)allow the racks (30) to be slid out of and into the stacked reactorassembly in case of e.g. maintenance or replacement.

The glass plates in the modules are stacked in such a way that they formcontinuous vertical channels from top to bottom of the stacked reactor.The separation baffle (7) between riser and downer parts runs over theheight of the glass modules, so that the space over the glass modulesand under is open. As a result, the reactor content (water and algaemixture) can circulate from riser to downer at the top, and from downerto riser in the bottom to create a closed loop.

While FIG. 6 shows two stacks of single racks, one serving as a riserand one as a downer part, more than two racks are equally feasible. Forexample, the reactor assembly may comprise three stacks, one as a riser,one as a downer, and the third one which can be switched off formaintenance or the like; in this case the valve (44) is a multi-wayvalve which ca be switched to either of the three or more stacks.

CO₂ can be injected in the air or gas mixture which drives thecirculation loop and ensures the stripping of oxygen produced in thereactor. There is a level control in the reactor (not shown) and allwater lost by harvesting of the algae and due to evaporation is pumpedinto the reactor at any point (total mixed system). Nutrients are fedinto the reactor together with the make-up water.

1. A set of two or more rectangular double glass plates having a layerof light-scattering non-uniformities or light-scattering particlesbetween single glass plates, the single glass plates having a thicknessfrom 5 to 50 mm, a length between 1 and 4 m, and a width between 0.5 and2.5 m, the two or more double glass plates being mounted in parallel ata distance between 10 and 200 mm, the distance being provided by glassstrips having essentially the same length as the rectangular doubleglass plates and being arranged along and fixed to only the long sidesof the double glass plates.
 2. The set of double glass plates accordingto claim 1, wherein the light-scattering non-uniformities or particleshave an average size between 0.2-1.2 μm.
 3. The set of double glassplates according to claim 1, wherein the light-scattering particles arefacetted particles having an average size between 10 μm to 500 μm,acting as geometric scatterers.
 4. The set of double glass platesaccording to claim 1, wherein the two or more double glass plates aremounted at a distance of between 20 and 150 mm.
 5. The set of doubleglass plates according to claim 1, wherein the glass has an iron contentbelow 0.04 wt. % (as Fe₂O₃).
 6. A rack in which a plurality ofequidistant sets of double glass plates according to claim 1 aremounted, the rack having means allowing the multiple arrays to bestacked in a reactor vessel.
 7. The rack according to claim 6, whereinthe plurality is 2-20 sets of double glass plates.